Apparatus and method for determining failure mode in a shear or pull test device

ABSTRACT

A method and apparatus for determining a mode of failure of a bond between an electrically conductive ball deposit and a substrate when breaking the ball deposit off of the substrate. The method and apparatus utilize tool force and displacement values to plot a force/displacement curve. The force/displacement curve is used to calculate the energy necessary to break the ball deposit off of the substrate. The energy value of a portion of a force/displacement curve is selected by reference to a peak force. In one preferred embodiment, this energy value is compared with a predetermined reference energy value to indicate a mode of failure. The peak force is preferably the maximum peak force, hi the preferred embodiment, the method and apparatus distinguish between a ductile failure mode and a brittle failure mode.

This invention concerns apparatus and methods for testing the strengthof a bond in a semi-conductor device, and more particularly the strengthof a bond between a substrate and a means of electrical connectionthereto, typically a part-spherical deposit. Such deposits can be ofsolder, gold or other materials and are sometimes referred to as solderbumps or ball grid arrays.

Semiconductor devices are very small, typically from 0.2 mm square to 25mm square. These devices have sites for the bonding of electricalconductors thereto. Sites typically comprise part spherical electricallyconductive deposits of for example gold or solder, collectively known asballs, which in use have the appearance of a squashed sphere or lowcircular dome, and a diameter in the range 50-1000 μm. These depositsform part of the electrical path between, for example, a printed circuitboard and a chip, and may directly connect components, or may be joinedto a conductor which is itself connected to another component. Many suchballs may be provided as a regular grid-like array on a substrate.

Discrete balls are typically applied to a substrate and reflowed duringsubsequent connection to another component.

It is necessary to test the mechanical strength of the bond between theball deposit and the substrate in order to give confidence that theproduction bonding method is adequate, and that the bond strength issufficient. One kind of test applies a shear load to the ball deposit bymeans of a shear tool of a shear test machine in which the substrate issecured. Another kind of test applies a pulling load to the ball depositby means of a gripper of a pull test machine to which the substrate issecured.

There are two principal types of failure modes which occur during thesetypes of test: ductile failure mode and brittle failure mode. Breaking astrong bond will result in ductile failure mode of the ball deposit,with progressive deformation of the ball deposit until the ball depositis broken with part of the ball deposit often remaining adhered to thesubstrate. Breaking a weak bond will typically result in brittle failuremode with the ball deposit more or less cleanly tearing away from thesubstrate, leaving little residue adhered thereto. Consequently,inspection of the substrate after such a test can often indicate themode of failure. It would be advantageous to provide test machine userswith a test machine which could monitor parameters of the test tool andprocess those parameters to indicate automatically to the user thenature of the failure mode, and particularily whether a ductile orbrittle failure mode. These “principle” failure modes can be subdividedinto many other classifications. For example a brittle fracture canoccur at the bond between the ball deposit and the conductive pad it isadhered to or between the conductive pad and the substrate that it ismounted to.

A known shear test apparatus comprises a machine having a supportsurface and a test head movable in a controlled manner relative to thesupport surface. The test head carries a cartridge specific to the testto be performed and having one of several interchangeable shear toolsthereon. Typically the shear tool will be sized and/or shaped to suitthe ball deposit to be tested. In use, the substrate to be tested isattached to the support surface of the machine, and the tool is mountedinto the cartridge and driven against the ball deposit to perform therequired test, which may be for example a shear test or a reciprocatingfatigue test. Typically the tool moves against a stationary deposit.

A known pull test machine is similar and carries a cartridge having agripper adapted to the size and shape of the ball deposit to be testedand operable to exert a pulling load on the ball deposit substantiallyorthogonally to the substrate.

Although shear tests and pull tests are somewhat different, theforce/displacement characteristics are similar and can be used toclassify the failure mode, particularly whether the failure mode isductile or brittle.

A typical tool is small. The cartridge upon which the tool is securedhas a flexible element on which is mounted one or more force gauges(such as strain gauges). Thus, the force between the tool and balldeposit is measured at a distance by deflection in the flexible elementsof the cartridge. WO-A-2005/114722 shows an example of such a cartridge.The cartridge shown in WO-A-2005/114722 can be used in the Dage Model4000 Series machine available from Dage Precision Industries Ltd., ofAylesbury, United Kingdom. This machine typically measures the peakforce necessary to break a ball deposit off a substrate. Although themachine is able to measure peak force, it is difficult to determinewhether the bond between the ball and substrate has failed as a resultof ductile failure mode or brittle failure mode based on measurement ofpeak force alone. This difficulty arises from the fact that the peakforces in both brittle and ductile failure modes may be broadly similarfor a given size and shape of ball deposit.

It would be desirable to provide a method and apparatus fordistinguishing the mode of bond failure, and particularly to distinguishbetween the brittle and ductile failure modes and any other modes thatthese principle modes are subdivided into. As mentioned above, ifbreaking a ball off a substrate produces a ductile failure mode, thatindicates that the bond between the ball and the substrate is a goodbond, whereas a brittle failure mode would indicate that the bond issuspect and may be poor. Thus, providing the user of the machine withthe an output displayed on the machine monitor, for example, whichindicates to the user whether the failure mode was ductile or brittle,indicates to the user whether the bond tested was a good bond or a poorbond.

According to a first aspect of the invention there is provided a methodof determining the mode of bond failure when an electrically conductiveball deposit bonded to a substrate is subjected to a force to break theball deposit off of the substrate, the method comprising the steps of:

-   -   utilizing a tool of a bond testing machine to break the ball        deposit off of the substrate,    -   measuring the force applied by the tool to the ball deposit        while the tool is breaking the ball deposit off of the        substrate,    -   measuring the displacement of the tool relative to the bond        substrate while the tool is breaking the ball deposit off of the        substrate,    -   determining a selected peak force applied by the tool to the        ball deposit,    -   utilizing the force and displacement measurements to determine        the associated value of energy absorbed by the ball deposit from        the tool, the energy being determined with reference to said        selected peak applied force, and    -   comparing said associated value of energy to a reference value        of energy to indicate a mode of failure.

The method permits the mode of failure to be determined which has notbeen possible with prior art methods. The tool may be a shear tool forapplying a shear force to the ball deposit, or a gripper for applying anorthogonal pull force to the ball deposit.

In determining the energy absorbed by reference to the peak appliedforce, the value of energy absorbed before said selected peak appliedforce and/or the value of energy absorbed after said selected peakapplied force may be used as said associated value in the comparisonstep.

In a preferred embodiment, the value of energy absorbed after saidselected peak applied force is used as said associated value, and iscompared with a reference value to determine one of two failure modes,where one failure mode corresponds to an energy value below saidreference value, and the other failure mode corresponds to an energyvalue above said reference value.

The selected peak applied force may be the sole peak applied force. Themethod may include the further step of determining which of a pluralityof peaks of applied force is selected as said selected peak appliedforce.

In this refinement the selected peak applied force may not be themaximum peak applied force, but a preceding or succeeding peak whichbetter permits a mode of failure to be distinguished.

The method may further include the step of determining whether one of aplurality of peaks of applied force should be discarded for the purposeof determining said selected peak applied force.

Such a method permits the effect of transient or very small peaks to beeliminated. One method of distinguishing such peaks is to provide afilter adapted to eliminate peaks of less than a predeterminedsignificance, for example to eliminate peaks for which the nextsucceeding minimum has a value greater than a given percentage of thevalue of that peak, typically greater than 90%.

According to a second aspect of the invention there is providedapparatus for applying a load to an electrically conductive ball depositon a substrate, and comprising:

-   -   a support for holding a substrate,    -   a tool for applying a breaking force to an electrically        conductive ball deposit on the substrate,    -   a force detector for generating a force output corresponding to        the force applied by the tool to the ball deposit while the tool        is breaking the ball deposit off of the substrate,    -   a displacement detector for generating a displacement output        corresponding to the displacement of said tool with reference to        said support while the tool is breaking the ball deposit off of        the substrate,    -   a processor adapted utilize said force and displacement outputs        to determine the absorption of energy by said ball deposit        during breaking,    -   said processor being additionally adapted to determine the        portion of energy absorbed by said ball deposit by reference to        a peak of transient force detected by said detector,    -   and said processor being still further adapted to indicate a        mode of failure by comparing said portion to a reference value.

The load applied to the ball deposit may be via a shear tool or via apull tool, as previously described.

The energy absorbed by the ball deposit is in one embodiment ofapproximately the same magnitude for both failure modes up to the pointof maximum applied force. It has been observed by the applicant,however, that the energy absorbed after the point of peak applied forcemay be markedly different. Accordingly, a determination of this postpeak applied force portion of absorbed energy can be used to determinethe mode of failure by reference to a pre-determined energy value. Thispredetermined energy value can be determined empirically by repetitivetests, taking into consideration the bonding method and size of the balldeposit.

An advantage of the invention is that existing methods of measurement ofapplied force can be utilised in conjunction with displacementmeasurement, so as to permit determination of a force/displacementcharacteristic, and thus determination of a value representing theportion of energy which is absorbed after the point of maximum appliedforce. This value need not be determined absolutely, but cannevertheless be compared with a suitable reference value to indicatewhether failure is by ductile or brittle fracture. The value may also beprovided as an absolute value, where required.

It will be appreciated that the comparison may indicate whether themeasured value is above or below the reference value, in order to allowan appropriate conclusion to be drawn. A determination that the measuredvalue is above the reference value indicates ductile failure modewhereas a determination that the measured value is below the referencevalue indicates brittle failure mode, since producing a ductile failuretakes more force than producing a brittle failure.

In the case of plurality of peaks of applied force, the apparatus may beadapted to distinguish said peaks, and eliminate any peak which isconsidered insignificant by reference to a predetermined standard.

In a refinement, the method and apparatus takes into account speed. Thustests may be performed at a range of impact speeds of a shear tool uponthe stationary ball deposit. The measurements from these tests can beused to determine the mode of failure. For example, it may be shown thatfor a given bonding method, the frequency of brittle fracture mayincrease as speed of movement of the tool increases. Brittle failuremode will occur at lower speeds than ductile failure mode. It isdesirable to select a value of speed which best distinguishes betweenbrittle and ductile failure.

Several methods may be used to determine an optimum tool speed. Thespeed should not be so low as to allow the majority of balls to exhibitductile failure, nor so high as to cause inevitable brittle failure.What is required is a speed which allows all acceptance bonds to exhibitductile failure, and this speed can for example be determinedempirically from an examination of broken bonds.

Preferably the method of the invention includes the step of applying theforce at a speed selected to ensure that a satisfactory bond between theball and component exhibits ductile failure, whereas an unsatisfactorybond exhibits brittle failure.

The apparatus of the invention may further include variable speed meansfor applying said force to said ball, said variable speed means beingselectable in the range 0.5 m/sec. to 10 m/sec.

In a preferred embodiment the apparatus includes an electronic analysismodule for receiving digital signals representing changes in appliedforce and displacement with time, and adapted to integrate and otherwiseprocess such signals to produce values of energy absorbed, peak appliedforce and rate of change of force, and to process said values todetermine a mode of failure, in some embodiments utilizing pre-setparameters.

Other features of the invention will be apparent from the followingdescription of a preferred embodiment illustrated by way of example onlyin conjunction with the accompanying drawings, in which:

FIG. 1 shows in side elevation an array of solder balls on a substrate.

FIG. 2 a illustrates schematically a shear test.

FIG. 2 b illustrates schematically a pull test.

FIG. 3 illustrates schematically a prior art bond testing machine.

FIG. 4 shows a force/displacement characteristic for brittle failure,and

FIG. 5 shows a force/displacement characteristic for ductile failure.

FIG. 6 shows a bar chart comparison of energy thresholds.

FIGS. 6 a-6 d indicate force/displacement characteristics representativeof modes of failure indicated by the energy values of FIG. 6

FIG. 7 shows a force/displacement characteristic for pad crateringfailure

FIGS. 8-8 d show comparative force/displacement characteristics ofductile and pad cratering failure.

FIGS. 9 a-9 b show similar but distinguishable force/displacementcharacteristics of pad cratering failure.

With reference to the drawings, an array of solder balls 10 is providedon a substrate 11. The balls are typically in the diameter range 0.1-1.0mm, and closely spaced. The balls may be in a linear array, or may forexample be provided on a two-dimensional grid.

Bond strength at the ball/substrate interface may be tested in shearusing apparatus illustrated schematically in FIG. 2 a. A shear tool 12,which may be shaped to conform to the curved surface of ball 13, isapplied to the ball in the direction of arrow 14. Force is graduallyincreased until the ball shows ductile or brittle failure. Bond strengthmay be tested in tension using apparatus illustrated schematically inFIG. 2 b. A pull tool 20 comprises opposite gripping jaws 21 which maybe lowered and closed about a ball deposit 13 in the direction of arrows22, and then used to exert a pull force in the direction of arrow 23.

FIG. 3 schematically illustrates a known shear testing apparatus whichis adapted to sense and record the force required to shear a balldeposit off of a substrate. Such a system typically utilizes straingauges which produce an electrical output proportionate to the shearforce. The commercially available Dage Model 4000 Series machine,previously mentioned, is an example of such a testing apparatus.

In the machine of FIG. 3 the shank 23 of a tool holder 21 which holds ashear tool 12 is secured in a chuck 102 which is in turn mounted on atool mover 104. Tool mover 104 provides movement in the X direction, forexample, of the shear tool 12 to shear a ball deposit 13 off substrate11 and movement in the Z direction to vertically position the shear tool12 with respect to the ball deposits 13. Substrate 11 is mounted intable 118 which provides for movement of the substrate in the X and Ydirections relative to shear tool 12. Tool mover 104 is secured to ahousing 105 upon which is mounted a high-powered microscope 106. Housing105 can also include the processor which processes the electricalsignals received from a force detecting piezo-electric crystal 19 andpreferably displays the result of that processing on a display screen107 which is attached to the housing 105. The machine 100 also includesjoystick controls 106, 108 which move the X-Y table 118 and shear tool12. The operator looks through the high-powered microscrope 106 at thearea of the substrate 11 of interest and uses the joy sticks 106, 108 toposition the shear tool 12 adjacent to the ball deposit 13 to be shearedoff of the substrate 11. Once the shear tool 12 is properly positionedwith respect to the ball deposit 13, the tool mover 104 moves the tool12 a desired distance in the X direction, at a desired speed, to shearthe ball deposit 13 off the substrate 11. During this shear event, thepiezo-electric crystal 19, in this example, experiences forces oftension and compression and produces an electric signal from the crystal19 which can be correlated to the shear force required to shear the balldeposit 13 off if the substrate 11.

The electric signal produced by the piezo-electric crystal can beconveyed by insulated wires (not shown) to the processor containedwithin housing 105.

The shear tool 12 travels at a predetermined height above the substrateso as to eliminate dragging forces, and to ensure repeatability.

The machine of FIG. 3 is adapted to a pull test by the use of a suitablegripper tool, and by using the tool mover 104 to move the tool a desireddistance in the Z direction, at a desired speed to break the balldeposit from the substrate. Suitable force measuring strain gauges areincorporated in the tool or in the cartridge to which the tool isattached.

Typical characteristics of ductile and brittle failure are illustratedin FIGS. 4 and 5 by reference to shear tests; similar characteristicsare obtained in relation to pull tests. In both FIG. 4 and FIG. 5 theforce (F) applied by the tool to the ball deposit is increased along thevertical axis of the graph, displacement (D) of the shear tool occursalong the horizontal axis of the graph as the ball exhibits acombination of elastic (recoverable) deformation and ductile or brittle(unrecoverable) deformation. This combination effect causes a slightcurvature to the shape of the graph on the upslope 200, in contrast tothe common straight characteristic of pure elastic deformation. Thegraphs illustrated are somewhat simplified in order to show a smoothedtransition as failure occurs.

In both ductile and brittle failure mode, the energy absorbed up to thepoint 202 of maximum applied force is about the same, as can be seen bya comparison of the areas marked C and A. Energy is a function of forceand displacement and therefore can be represented by the area underthese curves.

However, after the point 202 of maximum force, as the ball or bond isfailing, the energy absorbed is significantly different, as representedby areas marked D and B.

Accordingly, in one embodiment, an electronically determined comparisonof these areas D and B can give a qualitative assessment of the mode offailure by reference to a pre-determined reference value. The referencevalue may be determined empirically by performing a series of tests onballs of the same size and bonding method, and noting the mode offailure. The reference value may for example be approximately half waybetween the energy values of areas D and B. In this example, area Drepresents a brittle failure mode and area B represents a ductilefailure mode in that more energy is required to produce a ductilefailure mode (i.e. to shear a good bond) than a brittle failure mode(i.e. to shear a poor bond).

Any suitable linear displacement means may be used to sense displacementof the tool, and in conjunction with force measurement to permitcalculation of the absolute value of energy to failure, if desired. Asystem such as that shown in FIG. 3 could be used, for example. Forcecan be measured using strain gauges contained within the cartridge 104,which as previously mentioned, is illustrated in some detail inWO-A-2005/114722.

In this type of system, the servomotors adapted to give relativemovement between the tool and substrate in the X-Y direction willproduce co-ordinates of the actual tool position. From theseco-ordinates, and an internal computer clock, the displacement of thetool, and the speed thereof can be calculated. The test speed may beheld constant by a closed loop servo system close to the programmedspeed. The actual test speed (correcting for slight differences betweenthe programmed speed an the real speed) is measured from servo positionsensors and time data. The force time data is transformed to forcedistance by the product of the time data and the actual test speed in aknown manner.

These force and displacement measurements would be used to generate thecurves of FIGS. 4 and 5. Once these curves have been plotted, readilyavailable, or easily written, software algorithms could be used tocalculate the areas B and D and to compare those areas to an empiricallydetermined reference value. The software could be contained in, forexample, a computer (not shown) contained within the housing 105 of FIG.3. The computer could then produce an output displayed on monitor 107 toindicate to the user whether the shear test indicated a ductile failuremode or brittle failure mode. This information would indicate to theuser whether a shear test indicated good bond strength (ductile failuremode) or poor bond strength (brittle failure mode) for the bond beingtested.

The accuracy of this calculation is a function of the measurement repeatrate. Thus for example an adequate repeat rate may give 20 measurementsof force in calculating the area C. Each measurement could be at apre-set increment of displacement.

A less frequent repeat rate may be sufficient where the thresholdbetween areas D & B is easily detected. In contrast, a more frequentrate may be selected in a relatively fine threshold, where closelyadjacent measurements of force are required to identify the position ofthe peak force with sufficient precision.

In a refinement of the invention, the tool 12 is applied to the ball ata variety of impact speeds to determine the robustness of the bondingmethod, and particularly to demonstrate that at the appropriate impactspeed all good bonds exhibit ductile failure, or conversely that all badbonds exhibit brittle failure.

In a further refinement of the invention, the areas A-D may beindividually calculated and compared in order to demonstrate a sequenceof failure modes. Thus FIG. 6 shows an ascending level of energy valuescorresponding to pre-selected threshold appropriate to each portion A-Dof the graphs of FIGS. 4 and 5.

In FIG. 6 increasing post peak energy is represented by the arrow ‘E’and four failure modes R, S, T and V are represented by the horizontaldimensions. Thus the division between R and S represents the energyvalue at which a failure mode changes from mode R to mode S, and so on.

FIG. 6 a represents failure mode R where post-peak energy is equivalentto the shaded portion.

FIG. 6 b represents a different mode of failure S where post-peak energyis equivalent to the shaded portion.

FIG. 6 c and FIG. 6 d represent the further respective modes of failureT, V having successively greater post-peak energies.

In a chart of the kind represented by FIG. 6, the horizontal divisions(energy thresholds) may be refined according to other measurablefeatures of the particular failure mode.

FIG. 7 illustrates an actual force-displacement curve of the generalform illustrated in FIG. 5. FIG. 7 however indicates a so-calledpad-cratering failure which has a successive peak forces F₁ and F₂ onthe up-slope. The term “pad cratering failure” means a failure in whichthe electrically conductive pad to which the ball deposit is adheredlifts from the substrate taking a portion of the substrate with itleaving a corresponding crater in the afore mentioned substrate. Apad-cratering failure is one specific mode of failure into which testresults might be desirably categorised. To detect this failure, thesoftware algorithm utilized by the computer would monitor the shear toolforce sensors to look for a first lower peak F1 followed by a secondhigher peak F2. If this condition is detected, the computer would outputa message on monitor 107 indicating a pad cratering failure to the user.This information would be useful to the user because it indicates whatparts of the bond construction are the weakest, be it ductile, brittleor pad crater.

FIG. 8 a illustrates a conventional ductile failure with post-peakenergy E₂ being somewhat greater than pre-peak energy E₁. FIG. 8 a has asingle clearly identifiable peak energy.

FIG. 8 b shows the pad-cratering failure of FIG. 7 in which the second(maximum) peak is identified as the dividing point. In this analysispost-peak energy E₃ is rather similar to the post-peak energy E₂ of FIG.8 a.

In contrast FIGS. 8 c and 8 d show the same graphs as FIGS. 8 a and 8 b,but the dividing point is determined by the first peak of thepad-cratering example. Clearly in this case the post peak energy E₄ ismuch greater than E₂.

Thus in the case that two peaks are detected on the up stage (i.e. asecond peak higher than a first peak), such a failure mode could beclassified, and a corresponding mode established by comparison of energysubsequent to the first peak. In this example the mode is identified aspad-cratering. It will be understood that each actual test graph mayexhibit particular characteristics of for example peak values, peakvalues in a particular recognisable order, slopes before, after andbetween peak values, and so on. These values of these parameters can bedetermined, for example, by digital recording, and selected as requiredin order to characterise particular failure modes. Thus other failuremodes may be indicated by for example three or four peaks. A logicalmachine analysis may sort or pre-sort the failure data according toparticular parameters, so that a two peak signal is analysed from thesecond peak, whereas a three peak signal is analysed from the middlepeak.

For each analysis it is recognised that a certain value of ‘noise’ maybe present and/or that the measured values have an accuracy determinedby the sophistication of given measuring equipment. Suitable filters,preferably digital electronic filters, may be included to allowpredetermined reference values and peaks to be appropriately compared.

As indicated in FIGS. 9 a and 9 b, two similar graphs may bedistinguished if the successive peaks are separated by a minimum valuewhich is less than a pre-determined fraction of the first peak value.Thus in FIG. 9 a, the value Y₂ is greater than the corresponding valueY₁ of FIG. 9 a, compared with the first peak value X. In this exampleFIG. 9 a could be treated as a two-peak graph, whereas FIG. 9 b could betreated as a single peak graph (indicated by P). Variations are ofcourse possible, and it is envisaged that programmable options may beoffered with, for example, the threshold values of Y₁, Y₂ being settablein order to allow for variations in the kind of ball deposits undertest.

As indicated above, a comparison of total energy values may besupplemented by reference to additional factors such as peak energyvalue. Thus peaks within a settable range may be selected for analysis,whereas peaks outside this range may be excluded.

Energy and peak force value may be used together as a filter of failuremodes, in any suitable combination, along with pre and post peak slopevalues.

Another alternative embodiment, where relative values and/ordimensionless ratios. One dimensionless ratio which is suitable is thenumerical ratio of pre and post peak energy such as C:D of FIG. 4, orA:B of FIG. 5. This ratio could be used to indicate a ductile versusbrittle failure mode. Likewise force values F₁:F₂ of FIG. 7 may be usedto indicate a pad crater failure mode.

The force/displacement characteristics illustrated in the accompanyingdrawings relate specifically to shear tests using apparatus of thegeneral kind illustrated in FIG. 2 a. Similar characteristics areobtained from pull test apparatus of the general kind illustrated inFIG. 2 b, and allow failure mode to be classified in the same manner,particularly by reference to port-peak energy levels.

It is intended to be understood that this invention is not limited tothe embodiments described herein and that variants, obvious to thoseskilled in the art, can be made which are within the spirit of theinvention and scope of the apparatus and method claims appended hereto.

1. A method of determining the mode of bond failure of an electricallyconductive ball deposit on a substrate which is subjected to a breakingforce by a tool to break the ball deposit off the substrate by breakingthe bond therebetween, the method comprising the steps of: producingelectrical force signals which indicate the force on the tool while thetool is breaking the ball deposit off the substrate, producingelectrical tool displacement signals which indicate the displacement ofthe tool while the tool is breaking the ball deposit off the substrate;processing the electrical shear force signals to determine a selectedpeak breaking force, utilizing the electrical force signals andelectrical tool displacement signals to determine an associated value ofenergy absorbed by the ball deposit by reference to said selected peakshear force, and comparing said associated value of energy to areference value of energy to indicate a mode of failure.
 2. A methodaccording to claim 1 wherein said associated value is determined to beone of energy absorbed prior to said selected peak, and energy absorbedafter said selected peak.
 3. A method according to claim 1 wherein saidselected peak is determined to be the maximum applied force.
 4. A methodaccording to claim 3 wherein said selected peak is determined to be thesole peak.
 5. A method according to claim 3 wherein said characteristiccomprises a plurality of peaks of applied force, and the method includesthe step of discarding one or more peaks of lower significance.
 6. Amethod according to claim 5 wherein a peak is discarded if the nextsucceeding minimum has a value greater than a pre-determined percentageof the value of that peak.
 7. Shear testing apparatus for applying ashear force to an electrically conductive ball deposit on a substrate toshear the ball deposit off the substrate and indicate the quality of thebond between the ball deposit and the substrate, comprising: a supportfor holding the substrate, the substrate having one or more balldeposits bonded to the substrate, a shear tool mounted within said testapparatus for applying a shear force to an electrically conductive balldeposit on the substrate, a force monitoring detector for generatingelectrical force signals representative of the force applied by theshear tool to the ball deposit during at least a part of the time thatthe shear tool is shearing the ball deposit off of the substrate, adisplacement monitoring detector for generating electrical displacementsignals representative of the displacement of said shear tool byreference to said support during at least a part of the time that theshear tool is shearing the ball deposit off of the substrate, a computerprocessor adapted to receive said force signals and displacement signalsand to utilize said force signals and displacement signals to determinethe absorption of energy by said ball deposit during at least a part ofthe time that said ball is being sheared from said substrate by saidshear tool, said computer processor furthered being adapted to utilizesaid force signals to determine a peak force value, said computerprocessor still further being adapted to determine the portion of energyabsorbed by said ball deposit by reference to said peak force value, andsaid computer processor still further being adapted to utilize saidportion of energy to indicate a mode of failure experienced by said balldeposit as said ball deposit was sheared from said substrate.
 8. Pulltesting apparatus for applying a pull force to an electricallyconductive ball deposit on a substrate to break the ball deposit off thesubstrate and indicate the quality of the bond between the ball depositand the substrate, comprising: a support for holding the substrate, thesubstrate having one or more ball deposits bonded to the substrate, apull tool mounted within said test apparatus for applying a pull forceto an electrically conductive ball deposit on the substrate, a forcemonitoring detector for generating electrical force signalsrepresentative of the force applied by the pull tool to the ball depositduring at least a part of the time that the pull tool is breaking theball deposit off of the substrate, a displacement monitoring detectorfor generating electrical displacement signals representative of thedisplacement of said pull tool by reference to said support during atleast a part of the time that the pull tool is breaking the ball depositoff of the substrate, a computer processor adapted to receive said forcesignals and displacement signals and to utilize said force signals anddisplacement signals to determine the absorption of energy by said balldeposit during at least a part of the time that said ball is beingbroken from said substrate by said pull tool, said computer processorfurthered being adapted to utilize said force signals to determine apeak force value, said computer processor still further being adapted todetermine the portion of energy absorbed by said ball deposit byreference to said peak force value, and said computer processor stillfurther being adapted to utilize said portion of energy to indicate amode of failure experienced by said ball deposit as said ball depositwas sheared from said substrate.
 9. Apparatus according to claim 7 andfurther including a filter adapted to eliminate one or more peaks oftransient force which are of less than pre-determined significance. 10.Apparatus according to claim 7 wherein said calculator, discriminatorand comparator are provided by a computer having a user interface. 11.Apparatus according to claim 10 wherein said user interface is adaptedfor selection of said reference value.
 12. Apparatus according to claim10 wherein said interface is adapted for selection of one of a pluralityof said peaks of transient force.
 13. Apparatus according to claim 10wherein said interface is adapted for selection of said portion.
 14. Amethod of determining the mode of bond failure of an electricallyconductive ball deposit on a substrate which is subjected to a breakingforce by a tool to break the ball deposit off the substrate and breakthe bond therebetween, the method comprising the steps of: producingelectrical force signals which indicate the force on the tool during atleast a part of the time that the tool is breaking the ball deposit offthe substrate, producing electrical tool displacement signals whichindicate the displacement of the tool during at least a part of the timethat the tool is breaking the ball deposit off the substrate; processingthe electrical force signals to determine a selected peak breakingforce, utilizing the electrical force signals and electrical tooldisplacement signals to determine a first associated value of energyabsorbed by the ball deposit prior to said selected peak breaking forceand to determine a second associated value of energy absorbed by theball deposit after said selected peak breaking force, and comparing saidfirst and second associated values of energy to indicate a mode offailure.